6,7-Dihydro-2-methylpyrazolo[3,4-d]thiazolo[3,2-a]pyrimidin-4(2H)-one which is a tricyclic compound having a 6,7-dihydropyrazolo[3,4-d]thiazolo[3,2-a]pyrimidin-4-one skeleton and which has a substituent on a nitrogen atom at the 2-position is described in Liebigs Ann. Chem., 1121 (1987).